Most oxygen delivered to tissue is carried by hemoglobin. The proportion of oxygen delivered by hemoglobin may drop somewhat for a patient breathing very high levels of oxygen, however. A pulse oximeter may use the time-varying optical response of different wavelengths of light traveling through tissue to estimate pulse rate and the fraction of hemoglobin that carries oxygen molecules in the pulsatile portion. Each data point in the pulse oximeter's time series of optical response measurements represents the quantity of those photons of light that travel on a path from light emitter to light detector and are successfully measured by the detector. The signal level may be reduced by, for example, optical attenuation and scattering of tissue along the path from emitter to detector.
A pulse oximeter may measure changes in the amount of hemoglobin in a local area, along with the spectral response of the changes, over a cardiac cycle, without the need for an absolute reference. A photoplethysmogram (PPG) is an inverted, bandpass filtered graph of the optical response at one wavelength or a combination of wavelengths with respect to time. The ratio of pulse-synchronous change in absorption (PPG modulation) to average signal level is sometimes referred to as percent modulation, although this number depends on the relative quantities of different types of hemoglobin and other absorbers.
An optical sensor may be positioned at a tissue site on a subject that includes a concentration of smaller blood vessels. The light transmitted through these smaller blood vessels may decrease during the pressure wave of systole because of an attending greater volume of absorbers in the distended vessels of the vascular tree. The optical signal may rebound (e.g., increase) during diastole when lower pressure allows the amount of absorbers to decrease. Although the flow of fresh blood may take many seconds to travel to a peripheral site, the pressure waveform traverses the pressurized arterial tree within a much shorter time span. Vascular resistance and compliance of the small arteries, arterioles and capillaries reduces pressure and largely damps out any pulse-synchronous volume changes distal to these vessels.
Empirical calibrations of pulse oximeters using blood-gas measurements may include one or more assumptions that impact accuracy. For example, while tissue sites where pulse oximeters may be used may include arteries and arterioles as the primary conduit for delivering oxygen-rich blood from the left heart to tissue, these tissue sites generally comprise several other tissues and/or structures.
A number of factors may affect average optical response and pulse-synchronous modulation, particularly at peripheral sensor sites such as the finger, ear or foot. The body's control of circulation to the periphery is particularly used for temperature regulation, with vasodilation increasing peripheral blood flow to disperse heat and vasoconstriction acting to minimize the body's loss of heat. Local blood perfusion adapts based on tissue needs and metabolic activity. Drugs, therapies and shifts in fluid or electrolyte content may lead to changes in the distribution of systemic blood flow to the various organs, including the skin. This may also lead to changes in optical response and modulation. Shock, trauma or infection may cause adjustments in blood distribution. Changes in cerebral, muscular, renal or splanchnic circulation, hormonal activity and disease states also may have a significant influence on peripheral blood flow.
These factors may make it difficult to estimate the heart's total stroke volume based on the measurement of blood volume changes at a single sensor site. A “perfusion index” may be used on some occasions, but such a parameter may not adequately account for the myriad influences on distribution of blood flow to different parts of the body. Thus, a perfusion index at a peripheral sensor location may not provide a direct indication of stroke volume and cardiac output, nor even the adequacy of tissue perfusion at the sensor site.